In use, one for example places a contact stud provided with a gold layer on the aperture device in order to measure the thickness of the layer of gold. The thicknesses of the films falls in the micrometric region. In order to be able to obtain accurate and reproducible measurement results it is necessary to position the parts very exactly on the aperture. The size of the passage in the case of small apertures amounts to 4/10 mm. If the part to be measured wobbles or if it becomes displaced by even a few micrometers, then the measured value automatically changes by about 30%.
The change in the spacing of the part from the surface on which it is lying on the aperture gets included in the measurement.
Obviously, changes in spacing cannot essentially be avoided when the product to be measured is not--as in the above case--non-moving with respect to the aperture but rather is a product under measurement which is being moved. On account of unavoidable changes in the thickness of the product being measured and because of the values corresponding to the extent of shaking in the case of products which are being moved, measurement is very difficult.